The present invention relates to fluid jet printing and, more particularly, to a fluid jet printer in which printer operation and reliability during start-up and shutdown are enhanced. Specifically, the present invention provides a means for reducing fluid contamination of charge electrodes by drops from unstable jet drop streams.
Fluid jet printers typically include a print head, defining a fluid manifold or reservoir, to which electrically conductive fluid, such as ink, is supplied under pressure. A plurality of orifices defined by an orifice plate communicate with the fluid reservoir. The orifices may typically be arranged in one or more rows. Ink is forced under pressure through the orifices and emerges as a plurality of fluid filaments. The filaments are caused to break up into streams of fluid drops of substantially uniform size and spacing by mechanical stimulation of the print head, by mechanical stimulation of the orifice plate, or by generating pressure waves which travel through the fluid in the fluid reservoir.
Charge electrodes are positioned beneath the orifice plate, adjacent the ends of the fluid filaments. Electrical charge potentials are selectively applied to the charge electrodes to induce corresponding electrical charges on the drops formed from the fluid filament tips. The charged and uncharged drops then pass downward through a deflection field, with the charged drops being deflected, and the uncharged drops passing unaffected through the field. Drops which are not to be deposited on a print receiving medium are caught by a drop catcher as a result of their trajectories, whereas the balance of the drops strike the print receiving medium and, collectively, form a print image thereon. Drops may be charged binarily or, alternatively, they may be charged to a plurality of charge levels, as required by the particular printer configuration.
During the start-up of the printer as the fluid pressure within the fluid reservoir is increased to an operating level, the fluid flow through the orifices and the formation of drops from the fluid filaments are extremely irregular and unpredictable. Exceptionally large drops of ink may be formed from the filaments, and the trajectories and velocities of such drops are relatively uncontrolled. As a consequence, problems have been experienced with prior art fluid jet printers during start-up involving wetting of the charge electrodes. Errant drops may accumulate on the electrodes, causing the charge electrodes to short out or interfering with the trajectories of the jet drop streams once stable jet operation is obtained. Similar problems are encountered during shutdown of prior art printers as the pressure of the fluid in the reservoir is reduced, and fluid flow through the orifices is terminated.
Several approaches have been taken to overcome the problems presented by jet instability at start-up and shutdown of a fluid jet printer. In IBM Technical Disclosure Bulletin, Vol. 20, No. 1, June 1977, pages 33, 34, a charge electrode structure is shown in which a notched charge plate is pivotally mounted to rotate about an axis parallel to the row of orifices in the print head and perpendicular to the jets. The charge plate defines a plurality of notches along one of its edges which are lined with electrically conductive material. Each lined notch acts as the charge electrodes. By providing such a notched configuration, the charge plate may be moved into its operating position, in which each of the notched electrodes partially surrounds a corresponding fluid filament, after start-up is completed. Additionally, the charge plate may be moved to a retracted position prior to shutdown of the print head. By this technique, wetting of the charge electrodes is prevented.
In an alternative embodiment, the charge plate is mounted on a "cardo spring" and is translated into and out of its operating position by means of a cam contacting the end of the charge plate support structure. Movement of the charge plate in this embodiment is generally in the plane of the plate. The charge plate pivot embodiment is somewhat disadvantageous in that a substantial amount of space around the charge plate must be kept open to allow for the pivoting movement of the charge plate. The cardo spring embodiment, on the other hand, is disadvantageous in that movement of the charge plate may not be precisely controlled.
IBM Technical Disclosure Bulletin, Vol. 19, No. 8, January 1977, pages 3216, 3217, discloses an ink jet printer in which a pair of charge electrode plates are moved laterally into and out of operating positions after start-up and prior to shutdown, respectively. A similar charge electrode movement is shown in U.S. Pat. No. 4,238,805, issued Dec. 9, 1980, to Paranjpe et al. In both instances, notched charge electrode plates are translated into and out of operating positions in a direction perpendicular to the row or rows of jet drop streams. While such an arrangement permits a charge electrode plate to be retracted during start-up and shutdown, thus reducing substantially the possibility of fluid contamination of the charge electrodes, translational movement may be somewhat difficult to produce and to control precisely. Since the jet drop streams in a jet drop printer are typically positioned very closely together, on the order of 120 streams per inch along the row, it will be appreciated that the charge electrode grooves or notches are very small and closely spaced, and that very precise movement of a charge electrode plate into and out of its operating position must be produced to prevent the charge electrodes from contacting the fluid filaments.
U.S. Pat. No. 4,305,079, issued Dec. 8, 1981, to Mix, discloses an ink jet printer having a charge electrode plate which defines charge electrode notches along one edge thereof. The charge electrode plate is mounted on a bracket which, in turn, is pivotally mounted to the print head of the printer such that the charge plate may be pivoted about an axis parallel to the jet drop streams into a retracted position at start-up and shutdown of the printer.
The pivot mounting arrangement is such, however, that the direction of movement of the charge plate is not perpendicular to the edge of the charge plate and therefore not parallel to the sides of the notches. As a consequence, the notches must be sufficiently wide so that the sides of the notches do not touch the jets as the charge plate is pivoted. Therefore, the spacing between notched charge electrodes and adjacent jets produced by the print head may be too great for some printing applications.
Accordingly, it is seen that there is a need for a fluid jet printer and a method of printer operation in which a charge electrode plate is moved into and out of an operating position after start-up and prior to shutdown, respectively, in a reliable, precisely controlled fashion.